Prompted by the excellent results obtained by Phil Holland in England, and by the work reported by several course measurers on this forum, especially Mark Neal in this and other posts, I recently acquired a GPS unit and have been carrying out some experiments to determine how it may be used as an accessory during course measurement with a calibrated bicycle and Jones counter.
I am writing up my experience on a series of web pages which I hope will be useful to course measurers. the first of these pages, The Repeatability of Marked Way Points, is now complete and I welcome any comments from GPS users with more experience than me.
My Conclusion is:
Without any averaging, Mike's ETREX H could be used to record and then refind reference points normally (ie 95% of the time) to better than about 8 m. Occasional errors might be greater. At this level of performance the Etrex H is not accurate enough the reliably record the exact start or finish of a race, which still needs to be measured by means of a Jones counter and a calibrated bicycle wheel, and referenced with an offset from a nearby piece of fixed street furniture. However a GPS can be very useful in recording the general location of the reference points and for example it could fairly reliably distinguish between trees of similar appearance separated by more than 20m along a road side.
However a measurer should be aware that not every race director or indeed other measurers may have suitable equipment to be able to refind the references or indeed be able to operate it, as I discovered when I used grid refs to record the reference furniture locations for the mile splits for a recent country half marathon. When the organiser went out with a borrowed GPS to paint the mile marks, he could not master use of his GPS to locate the points, and he had to rely on the fact that I had provided a sufficiently comprehensive description of the location references (trees, gates, openings in hedges etc). That said, my record of the locations will certainly be useful aid to me if I have ever to go back and refind a lost reference. For example if a tree is cut down, I might be able to locate the correct stump, with the aid of my GPS. Occasionally I end up with paint marks on the road and no close by reference for a mile split. Here again, a GPS reference would usually be good enough to get me within a few metres so I could search for faint paint remnants.
Even for this limited use I think recording GPS way points at reference points will now be a regular part of my measuring procedure. There are however, other possible applications in course measuring which will be considered in other articles.
Edited 1 April 2009: I have now added some new pages which can be reached from http://coursemeasurement.org.uk/GPS/index.htm. These include a simulation of measuring courses of polygonal shape using my GPS to measure the locations of the corners. I come to the conclusion that used in this way my GPS has about half the accuracy of a course measurer with his calibrated bike and Jones counter. ie The GPS would pretty much always be within 20 m on a 10k polygon course.
Edited 2 April 2009: I have now added a page showing the tracks from 11 rides around my 4.5k Loop course in Abingdon. These tracks are quite close to one another, generally within 5 m of the mean track position. There are occasional divergences up to 10 m which last for a few hundred meters. See Repeatbility of 11 GPS tracks of the Abingdon 4.5k loop
Edited 5 April 2009: I have completed my first draft of pages which give the results over 16 rides round the Abingdon 4.5 k loop. The ETREX H gives very good results only about a factor of two worse than is typically obtained by an experienced measurer with a Jones counter. The final page of the series is HERE
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